Method and apparatus for drying paper on a yankee dryer

ABSTRACT

PAPERMAKING APPARATUS IS DISCLOSED WHICH INCLUDES STEAM SUPPLY MEANS ADAPTED TO APPLY STEAM TO THE SURFACE OF A WET PAPER WEB TO BE DRIED IN THE REGION IMMEDIATELY PRECEDING THE POINT WHERE THE PAPER WEB CONTACTS THE SURFACE OF THE YANKEE DRYER. ONE EMBODIMENT OF THE METHOD AND APPARATUS DISCLOSED INVOLVES CONTACTING A WET PAPER WEB WITH STEAM IMMEDIATELY BEFORE IT PASSES INTO THE ON-RUNNING SIDE OF A DRYING NIP FORMED BY A PRESSURE ROLL IN CONTACT WITH THE YANKEE DRYER. EMBODIMENTS OF THE METHOD AND APLPARATUS ARE DISCLOSED IN WHICH THE ABOVE FEATURE IS UTILIZED IN CONJUNCTION WITH SUCTION PRESSURE ROLLS, AND SOLID ROLLS, SUCH AS BLIND DRILLED ROLLS AND GROOVED ROLLS. A FURTHER EMBODIMENT IS DISCLOSED IN WHICH STEAM IS ALSO APPLIED TO THE EXPOSED SURFACE OF A WET PAPER WEB CARRIED ON A YANKEE DRYER BETWEEN TWO SUCCESSIVE PRESSURE ROLLS. ONE DISCLOSED FORM OF THIS EMBODIMENT INVILVES THE USE OF A HOOD BETWEEN THE PAPER WEB AND THE FELT LOOP RUNNING FROM ONE PRESSSURE ROLL, AROUND A GUIDE ROLL AND TO A SUCCESSIVE PRESSURE ROLL. ANOTHER DISCLOSED FORM OF THIS EMBODIMENT INVOLVES ENCLOSING THE ENDS OF THE SPACE DEFINED BY THE FELT LOOP AND THE SURFACE OF THE YANKEE DRYER AND THE INSERTION OF STEAM INTO THAT SPACE TO ELEVATE THE VAPOR PRESSURE INSIDE THE SPACE AT LEAST TO THE VAPOR PRESSURE OF THE WET PAPER WEB CARRIED ON THE YANKEE DRYER AND, PREFERABLY, SUBSTANTIALLY TO ATMOSPHERIC PRESSURE.

Feb. 2, 1971 c, DOUGLAS ET AL 3,560,333

METHOD AND APPARATUS FOR DRYING PAPER ON A YANKEE DRYER Filed Aug. 15, 1967 6 Sheets-Sheet 1 INVENTORS.

DEAN C. DOUGLAS RICHARD L. ELDERKIN DONALD F. POHLMAN AT ORNEY.

Feb. 2, 1971 D. DOUGLAS ETAL 3,560,333-

METHOD AND APPARATUS FOR DRYING PAPER ON A YANKEE DRYER Filed Aug. 15, 1967 6 Sheets-Sheet 2 /f' If Fly. 2

INVENTORS. DEAN C. DOUGLAS RICHARD L. ELDERKIN DONALD F. POHLMAN A TORNEY.

Feb. 2, 1971 D. c. DOUGLAS ETAL 3,560,333

METHOD AND APPARATUS FOR DRYING PAPER ON A YANKEE DRYER Filed Aug. 15, 1967 6 Sheets-Sheet 3 INVENTORS. DEAN C. DOUGLAS RICHARD L. ELDERKIN DONALD F. POHLMAN ATT NEY.

Feb. 2, 1971 D. c. DOUGLAS ETAL 3,560,333

METHOD AND APPARATUS FOR DRYING PAPER ON A YANKEE DRYER Filed Aug. 15, 1967 6 Sheets-Sheet 4 Fig. 5

INVENTORS. DEAN C. DOUGLAS RICHARD L. ELDERKIN DONALD F. POHLMAN ATTOR Y.

Feb. 2, 1971 D. c. DOUGLAS ET AL 3,560,333

METHOD AND APPARATUS FOR DRYING PAPER ON A YANKEE DRYER Filed Aug. 15, 1967 6 Sheets-Sheet 5 I! I, ""fi I Hg. 7

INVENTORS. DEAN C. DOUGLAS RICHARD L. ELDERKIN DONALD F. POHLMAN ATTO NEY.

Feb; 2, 1971 c; DOUGLAS ET AL 3,560,333

METHOD AND APPARATUS FOR DRYING PAPER ON A YANKEE DRYER IOO HEAT FLOW THROUGH DRYER SURFACE E 30 p I Q: 3 g a ":5 [1) (D Q 5 c O I P INVENTORS. DISTANCE ALONG DRYER CIRCUMFERENCE DEAN C. DOUGLAS (INCHES) RICHARD L. ELDERKIN v DON LD F. POHLMAN F/g 9 BY AT ORNEY- United States Patent Oflice Patented Feb. 2, 1971 3,560,333 METHOD AND APPARATUS FOR DRYING PAPER ON A YANKEE DRYER Dean C. Douglas, Wallingford, Richard L. Elderkin,

Media, and Donald F. Pohlman, Springfield, Pa., assignors to Scott Paper Company, Delaware County, Pa., a corporation of Pennsylvania Filed Aug. 15, 1967, Ser. No. 660,644 Int. Cl. D21f 11/00 US. Cl. 162206 I 19 Claims ABSTRACT OF THE DISCLOSURE Papermaking apparatus is disclosed which includes steam supply means adapted to apply steam to the surface of a wet paper web to be dried in the region immediately preceding the point where the paper web contacts the surface of the Yankee dryer. One embodiment of the method and apparatus disclosed involves contacting a wet paper web with steam immediately before it passes into the on-running side of a drying nip formed by a pressure roll in contact with the Yankee dryer. Embodiments of the method and apparatus are disclosed in which the above feature is utilized in conjunction with suction pressure rolls, and solid rolls, such as blind drilled rolls and grooved rolls. A further embodiment is disclosed in which steam is also applied to the exposed surface of a wet paper web carried on a Yankee dryer between two successive pressure rolls. One disclosed form of this embodiment involves the use of a hood between the paper web and the felt loop running from one pressure roll, around a guide roll and to a successive pressure roll. Another disclosed form of this embodiment involves enclosing the ends of the space defined by the felt loop and the surface of the Yankee dryer and the insertion of steam into that space to elevate the vapor pressure inside the space at least to the vapor pressure of the wet paper web carried on the Yankee dryer and, preferably, substantially to atmospheric pressure.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to improvements in the drying of a wet paper web where a traveling felt carries the web through a drying nip between one or more pressure rolls and the surface of a Yankee dryer and, more praticularly, to a new and improved method and apparatus therefor.

(2) Description of the prior art It is a common practice in the drying of wet paper webs to carry the web on a traveling felt through a driving nip in which it is pressed into engagement with the heated surface of a Yankee dryer. Often a series of pressure rolls are disposed adjacent the surface of the Yankee dryer and successively press the traveling felt against the web to create successive drying nips with the Yankee dryer. Moisture in the web is heated and evaporated by heat supplied to the Yankee dryer by steam circulated through its interior and conducted through the dryer shell. Moisture is also removed from the web by the felt which absorbs water by capillary attraction due to its lower moisture content and fibrous structure. To facilitate flow of moisture from the web to the felt, a number of different types of pressure rolls are employed. For example, solid types of pressure rolls such as plain rubber rolls, blind drilled rolls, or grooved rolls have been employed, the last two types receiving moisture in openings or recesses in their felt contacting surface at the nip. Another type is a suction pressure roll in which a suction box on the interior of the roll operates through a perforated roll or shell and draws moisture from the web into the felt and into the shell.

In recent years, limitations on drying capacity have caused serious restrictions upon the operating speeds of papermaking machines. To overcome these restrictions, manufacturers have made extensive efforts to increase the drying capacity of papermaking machines in a wide variety of ways. For example, Yankee drum dryers of larger diameter, on the order of 15 feet and more have been employed. In addition, attempts have been made to supply a greater amount of heat to the wet paper web by transferring more heat through the Yankee dryer shell. Pressure limitations of the material forming the dryer shell and the corresponding saturation temperature of the steam contained therein have been pushed upwards to the limits permitted by safety regulations. These and other efforts have generally required the use of steam at a higher temperature and pressure.

Attempts have also been made to remove a greater amount of moisture prior to the application of the wet paper web to the Yankee dryer, as by the use of a press section in which a suction pressure roll operating at a drying nip presses moisture from the web into a felt for removal. In a papermaking machine of the single-felt type, the wet paper web is normally removed from the Fourdrinier wire upon which it is formed and is transferred by a pickup felt directly to the surface of a Yankee dryer where it is pressed into contact with the Yankee dryer by a suction pressure roll. Attempts have been made to maximize the performance of the suction pressure roll, as by varying the nip pressure between the pressure roll and the Yankee dryer, by increasing the pressure drop through the web and the felt, and by increasing the length of felt subjected to the pressure drop at any given time. Where a two-felt machine is utilized, attempts have been made to improve the operation of the press section to maximize the amount of water transferred from the wet paper web into the felts. Successes achieved by these attempts have been generally quite small.

One of the problems experienced in the operation of both single-felt papermaking machines and two-felt papermaking machines has been the loss of heat from the outer surface of a Yankee dryer in the drying nips formed by one or more pressure rolls. It is believed that this is chiefly caused by contact of the relatively cold wet paper web and its carrying felt with the surface of the Yankee dryer. Heat removed from the dryer shell at these points is inefficiently used as it does not accomplish much evaporation and has little effect on the pressing operation. Furthermore, it is unavailable for drying the web in the remaining portion of the dryer circumference, where the greatest amount of evaporative drying preferably takes place. A number of attempts have been made to reduce this heat loss but most have been rather ineffective. For example, heated shower water has been employed for cleaning the felts before contact with the paper web in an attempt to apply heat to the felt. However, much of the heat gained by the felt is lost before it contacts the dryer and the cold web is not affected in any substantial way. The most that has been gained is a slight increase in pressing efficiency due to reduction of the viscosity of the entrained moisture, promoting its flow from the web into the felt.

It is an object and advantage of the present invention to provide improvements in the drying of a wet paper web.

It is a further object and advantage of the present invention to provide a method and apparatus for increasing the drying capacity of a Yankee dryer, including one or more pressure rolls forming a drying nip with the Yankee dryer through which a traveling felt carrying a web to be dried is fed, and means for contacting the Web with steam prior to its passage through the drying nip.

SUMMARY OF THE INVENTION In the method of the invention, a freshly formed wet paper web is pressed into contact with a traveling felt upon which it is transported in supported condition and free from any interim compression directly to a cylindrical heat-conductive drying surface. The felt-opposite surface of the web is contacted with steam immediately prior to contact of the felt-opposite surface with the drying surface. The felt-facing surface of the web is subjected to a partial vacuum immediately prior to contact of the felt-opposite surface of the web with the drying surface, whereby steam is drawn into the web and generally through the 'web so as to cause flow of entrained moisture from the web onto the traveling felt. The felt-opposite surface of the web is then pressed into contact with the drying surface and subsequently removed from the drying surface.

In another embodiment of the method of the invention, the web is passed through a drying nip in contact with a traveling felt between a Yankee dryer and a solid pressure roll, and steam is arranged to flow along one surface of the web in a direction opposite to the direction of travel of the web and from a point adjacent the onrunning side of the drying nip to a point spaced therefrom and preceding the pressure roll. The steam is drawn through successive transverse portions of the traveling web and the traveling felt preceding and moving toward the pressure roll, whereby the temperature of each of the web, the felt, and moisture contained therein is elevated prior to passing through the drying nip, resulting in retention of heat in the Yankee dryer until emerging from said drying nip.

In another embodiment of the method of the invention, the web in contact with a traveling felt is passed through at least two successive nips between a Yankee dryer and one of at least two successive pressure rolls. Successive transverse portions of the web between the two successive drying nips and in contact with the Yankee dryer are contacted with steam, thereby inhibiting loss of heat from the Yankee dryer surface due to the evaporation of moisture from the contacted portions of the web. A further embodiment of this method includes separating the web and the felt from each other between the two successive drying nips to create a space and to expose the portion of the web carried in contact upon the surface of the Yankee dryer, and inserting steam into the space. In a still further embodiment of the method of the invention, the web at the on-running side of the first drying nip is also contacted with steam and a partial vacuum is created through the first of the pressure rolls to draw the steam through successive transverse portions of the traveling web and the traveling felt.

The apparatus of the invention includes a rotatably mounted drum dryer having a cylindrical heat-conductive drying surface, and a rotatably mounted suction pressure roll disposed adjacent the drying surface. The rotational axis of the pressure roll is substantially parallel to the rotational axis of the drum dryer, and the pressure roll forms a drying nip with the drying surface. Vacuum means are provided for reducing the pressure on the interior of the suction pressure roll. A traveling felt partially wraps the pressure roll and runs along a path through the drying nip between the pressure roll and the drying surface in a direction perpendicular to the drying nip. The traveling felt is adapted to convey a paper web from a remote position to the drying surface. In addition, steam supply means are disposed adjacent the on-running side of the nip into which the felt and web are advanced. The steam supply means is adapted to envelop a portion of the drying surface and a portion of the paper web carried on the surface of the felt with steam.

Another embodiment of the apparatus of the invention includes a second rotatably mounted pressure roll disposed adjacent the drying surface and spaced farther along the path of felt travel from the on-running side of the drying nip. The rotational axis of the second pressure roll is substantially parallel to the rotational axis of the drum dryer, and the roll forms a second drying nip with the drying surface. The traveling felt runs through the second drying nip in contact with the web carried on the drying surface.

In one embodiment of the invention, a guide roll is positioned between the first and second drying nips, and the portion of the traveling felt between these nips run about it, forming a felt loop separated and spaced from the paper web carried on the drying surface and exposing one surface of the paper web running between the first and second drying nips. Hood means are supplied which substantially enclose the portion of the paper web so exposed between the first and second drying nips. In addition, means are included for introducing steam into the hood means and into contact with enclosed portions of the paper web. The above arrangement of apparatus inhibits loss of heat from the drying surface due to evaporation of moisture from enclosed portions of the paper web.

Another embodiment of apparatus of the invention includes a rotatably mounted solid pressure roll disposed adjacent a cylindrical heat-conductive drying surface of a rotatably mounted drum dryer. The rotational axis of this pressure roll is substantially parallel to the rotational axis of the drum dryer, and the roll forms a drying nip with the drying surface. A traveling felt is included which partially wraps the pressure roll and runs through the drying nip between the pressure roll and the drying surface in a direction substantially perpendicular to the drying nip. Hood means are provided which extend from the on-running side of the drying nip to a point spaced therefrom and preceding the pressure roll. The hood means are adapted to substantially enclose successive transverse portions of the web carried on the traveling felt. Steam supply means are connected to the hood means and are adapted to introduce steam into the hood means in a manner whereby steam flows along one surface of the web carried on the traveling felt in a direction opposite to. the direction of travel of the web and from a point adjacent the on-running side of the drying nip to a point spaced therefrom and preceding the pressure roll. Suction means are disposed adjacent the felt opposite the hood means and are adapted to draw the steam through successive transverse portions of the traveling web and the felt which precede and are moved towards the pressure roll.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side elevation view of a paper machine of the single-felt type showing one embodiment of apparatus of the invention,

FIG. 2 is a schematic, fragmentary, greatly enlarged side elevation view of a portion of the drying section of the paper machine shown in FIG. 1.

FIG. 3 is a schematic, side elevation view of a paper machine of the double-felt type showing a further embodiment of apparatus of the invention, and

FIGS. 4 through 8 are schematic, fragmentary, greatly enlarged, side elevation views of a portion of the drying section of a paper machine showing further embodiments of apparatus of the invention.

FIG. 9 is a diagrammatic illustration comparing steam flow from the cylindrical heat-conductive drying surface of a Yankee dryer in conventional apparatus to that in apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, the illustrated Fourdrinier type paper making machine shown in FIG. 1 is of a type commonly referred to as a single-felt machine for the reason that a single felt is utilized to convey a paper web from the Fourdrinier wire to the surface of a Yankee dryer without any interim compression as by a press section. A comparable variant of the machine shown in FIG. 1 employs a forming roll or cylinder in place of the Fourdrinier section. Although capable of making various types of paper of conventional weights, the single-felt machine is generally used for making lighter basis weight papers which require less drying capacity, therefore allowing the machine to operate at reasonably high speeds. In accordance with the invention, it has been found that the speed of singlefelt machines can be substantially increased by effecting changes in its drying section to increase its drying capacity.

The machine comprises a Fourdrinier wire or fabric which is trained over a breast roll 11 and a couch roll 12. A plurality of vacuum boxes 13 are provided beneath the upper flight of the wire 10. A plurality of idler and guide rolls 14 are provided for the lower flight of the wire 10. One of the guide rolls 14 serves as a stretch roll in accordance with conventional practice for maintaining the wire 10 taut about the rolls about which the wire 10 is trained.

A paper stock headbox 15 of a conventional type is provided at one end of the upper flight of the wire 10 and has an outlet or slice 1 6 for introducing the stock onto the wire 10. Rotative means (not shown) are provided for moving wire 10 so that a paper web formed thereon by drainage of water from the stock is advanced upon the upper flight toward couch roll 12. The couch roll 12 is of a conventional open construction, and a suction box 17 is provided within it to remove excess moisture from the web and the wire 10 when the web formed is couched from wire 10.

The felt section of the single-felt machine comprises a band of felt 18 disposed about the guide rolls 20, upper couch roll 21, stretcher rolls 22, guide roll 23, and pressure rolls 24 and 25. Upper couch roll 21 is positioned to contact wire 10 generally at a point where it runs in contact with the lower couch roll 12, although it may be slightly above that position sometimes so that it bears against an unsupported section of wire 10. Force is applied to stretcher roll 22 for maintaining the felt band 18 in taut condition.

Pressure rolls 24 and 25 are arranged to form pressure or drying nips with a Yankee dryer 26, through which nips the felt 18 passes. Throughout the specification and claims, the term drying nip means the nip between a pressure roll and a Yankee dryer, through which a wet paper web in contact with a felt travels. The felt moves through the drying nip in a direction substantially perpendicular to the drying nip. A felt loop 27 is created by guide roll 23 to withdraw the portion of the felt band 18 between the pressure rolls 24 and 25 out of contact with the surface of the Yankee dryer 26 or with a paper web carried thereon. A creping doctor blade 28 is disposed in contact with the surface of the Yankee dryer 26 and adapted to crepe and remove a dried or partially dried paper web from the surface of the dryer 26. Alternatively, a flat sheet may be desired and the web is removed from the surface of the Yankee dryer 26 without creping. Additional drying equipment, if required, and calendering and winding equipment (not shown) are employed beyond the Yankee dryer 26' for treating and handling the paper web thus formed and dried.

In the preferred embodiment of the invention, on a single-felt paper machine of the type described above, pressure roll 24 is a suction pressure roll and comprises a rotatably mounted perforated cylindrical shell and a stationary suction box 30 disposed adjacent the interior surface of that shell. A save-all pan 19 is disposed adja cent roll 24 and is adapted to catch moisture thrown off roll 24. Steam supply means 31 are provided adjacent the on-running side of the nip between suction pressure roll 24 and the Yankee dryer 26. The steam supply means 31 may be in any one of a variety of different forms but is generally in the form of an elongate header adapted to emit and direct steam toward the first drying nip. Baffles are sometimes employed to control the application of steam along the length of the header to insure uniformity of steam flow and temperature at various points across the paper web being dried. The steam supply means 31 or header is connected to a source of steam (not shown).

FIG. 2 is an enlarged view showing in greater detail a portion of the drying section of the single-felt machine described above with reference to FIG. 1. The construction and disposition of the steam supply means 31 relative to the first drying nip between the suction pressure roll 24 and the Yankee dryer 26 is more clearly shown. Steam supply means 31 comprises a cylindrical header 32 having a plurality of holes 33 cut along its length through the portion of its sidewall generally facing the drying nip. The plurality of holes 33 are arranged gen= erally in one or more lines along header 32. The header 32 is connected to a source of steam, which source has not been shown, through one of its ends. Extending from the sidewall of header 31 on each side of the line of holes 33 are two seal plates, 34 and 35. The seal plate 34 extends from the header 32 to a point adjacent the surface of the Yankee dryer 26 and spaced therefrom by a small incremental distance or clearance so that it is just out of contact with the Yankee dryer 26. Similarly, the seal plate 35 extends from the header 32 toward suction pressure roll 24 to a point adjacent to but spaced from the surface of the web carried on the felt wrapping suction pressure roll 24 by a small incremental distance or clearance. Holes 33 are adapted to emit steam from the interior of header 32 into the space between seal plates 34 and 35 in a manner whereby the steam impinges upon the seal plates and is uniformly diffused. This prevents damage to the wet paper web due to jetting of steam against it.

Seal plates 34 and 35, together with the surface of Yankee dryer 26 and the supported web on felt 18, form an enclosed space or prenip region which may be enclosed by end plates (not shown) on each end. The majority of the steam inserted into the prenip region through the header 32 is drawn by the action of the suction box 30 inside the suction roll 24, into the web and the felt where it condenses.

Throughout the specification and claims, the term steam means the invisible gas or vapor into which water is converted when heated to the boiling point; that is, water in the state of vapor. Preferably, pure steam is utilized although the practicalities of generating and applying steam generally result in the admixture of some air with the steam. Steam supplied to header 32 is preferably saturated steam at about 240 F. and at a slight pressure, on the order of 10 p.s.i gauge. When emitted or throttled into the space between seal plates 34 and 35 where it is preferably at 0 p.s.i. gauge or at atmospheric pressure, it becomes superheated steam at about 230 R, which is capable of imparting a greater amount of heat to the web and the felt than is saturated steam at the same pressure.

In accordance with the invention, when the web carried on felt 18 moves past seal plate 35, all but a thin layer of cold boundary air is removed from contact with the web, and steam envelopes the web. The thin layer of cold boundary is drawn into and through the web and the felt 18, followed by steam which condenses upon the web and the felt 18, imparting heat thereto in the form of heat of vaporization. When the web and the felt 18 are pressed into contact with the surface 29 of the Yankee dryer 26, their temperatures more closely approach the temperature of the surface 29 and therefore they do not withdraw as much heat from it. In addition, the reduced viscosity of entrained moisture in the heated web and the heated felt 18 promotes its removal in the pressing operation since the heated moisture flows more readily.

With the arrangement of apparatus described above, it has been found that substantial increases in the speed of operation of single-felt papermaking machines are achieved without any sacrifice or reduction in the dryness level of the paper sheet produced. In other words, it has been found that the drying capacity of any given single-felt papermaking machine can be increased, allowing it to dry a greater amount of paper sheet in a given length of time. It is well known that most Yankee dryers are currently operated at the upper limits of their capacity in terms of heat transfer and in terms of steam pressure. Furthermore, condensate removal systems have been designed and constructed to maximize heat transfer through the dryer. Any increases in drying capacity due to changes in the construction of Yankee dryers over the past few years have been generally minimal.

With the invention, however, it has been possible to increase the speed of a Yankee dryer on a single-felt papermaking machine on the order of 800 feet per minute, that is, from a speed of 3200 feet per minute up to about 4000 feet per minute. Slightly lower increases have been experienced with the use of the invention in paper machines of the two-felt type.

In some of the embodiments of the invention, a forming roll type of papermaking machine has been employed instead of the Fourdrinier type described above. The forming roll type has generally been similar to that described in U.S. Pat. No. 3,252,853. With the use of a forming roll machine, the web transferred by the felt to the dryer generally contains a lesser percentage of water which does not thrust as great a burden upon the drying capacity of the drying section of a single-felt papermaking machine in this embodiment, however, it is important that the invention be utilized.

FIG. 3 illustrates a papermaking machine of the type commonly referred to as a double felt or tow felt machine for the reason that one felt known as a pickup felt is utilized to convey a paper web from the Fourdrinier wire or forming surface to the surface of a Yankee dryer while a second felt, commonly referred to as the wet felt," is utilized to contact the web and to remove moisture therefrom generally in conjunction with a press section which compresses the web between two felts as it passes through one or more pressure nips. Referring now to FIG. 3, it will be seen that the first portion of the machine, that is the Fourdrinier section, is the same as described above with reference to FIG. 1.

Similarly, a pickup felt band 36 is disposed about the guide rolls 37, upper couch roll 38, guide roll 40, stretcher roll 41, press rolls 42, 43 and 44 of the press section, and pressure rolls 45 and 46 of the dryer section. Again, upper couch roll 38 is positioned to contact wire generally at a point where it runs in contact with the lower couch roll 12, although it may be slightly above that position sometimes so that it bears against an unsupported section of wire 10. Stretcher roll 41 is one on which force is applied for maintaining the felt band 36 in taut position. Pressure rolls 45 and 46 are arranged to form pressure nips with Yankee dryer 26 through which nips the felt 36 passes. A save-all pan 57 is disposed adjacent pressure roll 45 and adapted to catch water thrown from roll 45. A felt loop 47 is created by guide roll to withdraw the portion of the felt band 36 bet-ween the pressure rolls and 46 out of contact with the surface of the Yankee dryer 26 or with a paper web carried thereon to form a felt loop 47.

A wet felt 48 is disposed beneath the pickup felt band 36 and is disposed about press rolls 50 and 51, guide rolls 52, and wringer rolls 53 and 54. Wringer rolls 53 and 54 are arranged to create a nip through which the wet felt passes and are adapted to remove moisture in the wet felt received from the wet paper web being pressed. The wet felt is pressed into engagement with the web by press roll 51 creating a nip with press roll 44. The paper web sandwiched between the wet felt and the pickup felt 36 is carried through the press section, indicated generally by reference numeral 55, comprising lower press roll 50 creating pressure nips with upper press rolls 42 and 43. In some embodiments, the lower press roll 50 may be a suction press roll. The arrangement of the drying section in the two felt machine is substantially the same as shown in FIG. 2 and the conditions of operation are practically identical therewith. Again, pressure roll 45 is a suction pressure roll and contains a stationary suction box 56.

In operation, a freshly-formed wet paper web is couched from the Foudrinier wire onto the pickup felt and transferred to the cylindrical heat-conductive drying surface of the Yankee dryer. The felt-opposite surface of the web, that is, the surface of the web facing away from and most remote from the felt, is contacted with steam immediately prior to contact of the felt-opposite surface with the drying surface. The felt-facing surface of the web, that is, the surface of the web which is adjacent and in contact with the felt, is subjected to a partial vacuum immediately prior to contact of the felt-opposed surface with the drying surface. This causes steam to be drawn into the web and generally through the web and causes entrained moisture to be drawn from the web into the traveling felt. The felt-opposite surface of the web is then pressed into contact with the drying surface and subsequently removed from the drying surface. In the operation of a single-felt papermaking machine as described above with reference to FIGS. 1 and 2, the web is transported on the pickup felt directly to the Yankee dryer without the application of any interim pressing operation or any other treatment for removing any amount of moisture from the web prior to the Yankee dryer and while carried on the pickup felt.

As indicated above, it has been found that a noticeable increase in the drying capacity of the two-felt papermaking machine has been experienced by the use of the invention in conjunction therewith. The increase in machine speed made possible through the use of the invention with two-felt machines, although not as great as in the case of single-felt machines, has been a substantial one, on the order of about 500 feet per minute, that is, from about 3300 feet per minute up to about 3800 feet per minute in a typical operation.

Where a two-felt papermaking machine is employed, utilizing interim compression in a press section to re move entrained moisture from the paper sheet before it is applied to the Yankee dryer, the requirements of the drying section of the papermaking machine have not been as great as in the single-felt machine. The effects of the use of steam supply means at the drying nip between the first pressure roll and the Yankee dryer have not been as great because the percentage of moisture in the web at that point is lower and heat losses from the Yankee dryer surface at drying nips are generally lower.

It is believed that the advantageous effects experienced by use of the invention in either a single-felt papermaking machine or a two-felt papermaking machine can be at tributed to two factors. Conventionally, the Yankee dryer surface transmits heat to the web during contact with the web to heat it to evaporation temperature so as to evaporate water within the web. By the application of steam in the pre-nip region, the water in the web and the felt is preheated which conserves heat in the Yankee dryer since less heat will be conducted into the web and the felt. Ideally, no heat will be lost at this point from the Yankee dryer surface. Therefore, the reserved heat in the Yankee dryer is available for evaporative drying purposes at subsequent points in the drying process. A Yankee dryer shell has a temperature gradiant across it which varies from the higher temperature on the interior surface, which is close to the temperature of the steam heating the dryer, to the lower temperature on the outer surface which the dryer is able to maintain taking into account the heat lost to heating and drying webs placed in contact with it. The temperature gradiant through the major portion of the dryer shell remains fairly constant while the temperature of the outer surface fluctuates somewhat as the dryer transmits heat. If the amount of heat lost by the dryer surface is reduced as it passes through the drying nips, the dryer surface has a higher exterior surface temperature for evaporative drying and a concomitant increase in dryer capacity. Additionaly, heating the water in the paper sheet and the felt prior to the drying nip reduces the viscosity of the water and promotes flow and removal of the water from both the paper sheet and the felt when subjected to pressure as within the nip.

It is believed that the difference in effect of the invention on a single-felt machine and a double-felt machine is partly due to the characteristics of the pickup felts employed. The pickup felt 1 8 on a single-felt machine is normally quite porous and allows a greater flow of air through it at a given pressure drop. For example, needled felts are often employed as distinguished from the woven felts which are often used as the pickup felt 36 on a double-felt machine. Therefor, more cold air is normally sucked through the web and felt prior to the first drying nip, resulting in a colder web and felt contacting the Yankee dryer surface than is the case on a conventional double-felt machine.

Although a preferred embodiment of apparatus of the invention has been described above with reference to the most typical arrangements of pressure rolls in combination with a Yankee dryer, and including the same on both a single-felt papermaking machine and a double-felt papermaking machine, it has also been found advantageous to utilize the method and apparatus of the invention in other embodiments which, with certain modifications described below, are capable of considerable improvement in drying capacity over that achieved at present with similar equipment. In most instances, however, the increase in drying capacity is less than that experienced with the embodiments shown in FIGS. 1 through 3 previously described. Each of the embodiments of the method and apparatus described below may be utilized to advantage with papermaking machines of either the singlefelt or double-felt type.

FIG. 4 illustrates an embodiment of apparatus of the invention in which a single suction pressure roll 60 is disposed adjacent the cylindrical drying surface 61 of a Yankee dryer 62 to create a drying nip 68. A suction box 63 is disposed inside suction pressure roll 60 and is arranged to draw air or steam along with moisture through a traveling felt 64 entrained over it. A saveall pan 59' is disposed adjacent roll 60 and adapted to receive moisture thrown from roll 60. The felt 64 is fed about a guide roll 65 and advanced into contact with the suction pressure roll 60. The traveling felt 64 carries a wet paper web on its outer surface which is pressed into engagement with the drying surface 61 of the Yankee dryer 62 and carried thereon to a circumferentially spaced position where it is removed. Steam supply means 66 are disposed adjacent the onrunning side 67 of the drying nip 68 in the manner described above. The steam contacts the exposed portion both of the cylindrical drying surface 61 of the Yankee dryer 62 and of the wet paper web preceding the drying nip 68 and is drawn through the paper web and the felt 64 to impart heat thereto and to remove entrained and evaporated moisture.

FIG. 5 illustrates an arrangement of apparatus similar to FIG. 4 in which suction pressure roll 60 has been replaced with a solid pressure roll 70, such as a blind drilled roll or a grooved roll. The steam supply means 66 includes a header 71 which extends along the length of solid pressure roll 70 and the Yankee dryer 62, and a hood formed from a seal plate 72 and a seal plate 69, which extends from a point adjacent the on-running side 67 of the drying nip 68 and the drying surface 61 of the Yankee dryer 62 to a point preceding the drying nip 68 and preceding the solid pressure roll 70. Steam supplied to header 71 flows into the drying nip 68 and returns along the surface of the paper web to a point preceding the pressure roll 70* in a direction opposite the direction of travel of the web and the felt 64. A suction box 73 is disposed adjacent the felt 64 on the side opposite to the hood and is adapted to draw steam through the web and the felt 64. A pair of rolls 74 and 75 are arranged at each side of hood 73 and in contact with the felt 64 so as to provide a moving seal for the hood 73. Steam enters suction box 73 through openings (not shown) in the portion of its sidewall between seal rolls 74 and 75. The above-described arrangement of apparatus enables heat to be applied to the web, the felt, and to moisture contained therein even where a solid pressure roll 70 is uti lized. This results in retention of heat in the Yankee dryer surface 61, which heat is utilized subsequently for drying the wet paper web.

FIG. 6 shows an alternative embodiment of the apparatus shown in FIG. 5 in which a second pressure roll 76 is included which may be of either the suction pressure roll type or the solid pressure roll type, a solid pressure roll being shown. A felt loop 77 is created between the first pressure roll 70 and the second pressure roll 76 by a guide roll 78 about which successive transverse portions of a traveling felt 64 are entrained while the web carried into the first drying nip 68 remains in contact with the cylindrical drying surface 61 of the Yankee dryer 62. Heat losses from the dryer surface 61 are minimized at the first drying nip between pressure roll 70 and dryer 62.

FIG. 7 illustrates a modified embodiment of the apparatus shown in FIG. 2, in which a hood 80 is disposed adjacent the cylindrical drying surface 29 of the Yankee dryer 26 between the first and second drying nips and within the reaches of the felt 18 forming the felt loop 27 about guide roll 23. Steam is applied through a duct 81 into the hood 80. The hood 80 is adapted to substantially enclose the portion of the paper web exposed upon the dryer surface 29 between the first and second drying nips formed by pressure rolls 24 and 25, respectively, with the Yankee dryer 26 and to contact such portion with steam. This has been found to inhibit the loss of heat from the dryer surface 29 due to the evaporation of moisture from the enclosed portions of the paper web. It also imparts heat to the web and the felt which promotes the flow of entrained moisture from the web into the felt during the pressing operation as the web passes through the second drying nip. In addition, the heat imparted to the web and felt reduces or eliminates the loss of heat from the dryer surface 29 as it passes through the second drying nip, which heat is then retained in the dryer surface 29 until utilized for evaporative drying as the dryer rotates past pressure roll 25.

FIG. 8 is a modified embodiment of the apparatus described in FIG. 7 in which the ends of the space defined by the felt loop 27 are enclosed by end plates 82 which utilize the felt 18 itself to form a hood which serves the same function as the hood 80 shown in FIG. 7. Steam is fed through a duct into one of end plates 82 so that the exposed surface of a web carried upon the cylindrical drying surface 29 of Yankee dryer 26 is contacted with steam thereby inhibiting the evaporation of moisture and loss of heat therefrom during the time the web is so enclosed.

In all of the above-described embodiments of apparatus of the invention, it will be apparent to those skilled in papermaking that high velocity dryer hoods (not shown) might be and generally are used to advantage with the Yankee dryer. The dryer hoods generally cover the major portion of the dryer surface following the last drying nip and circulate air over the surface of the web to remove vapors and to promote evaporative drying. Apparatus with and without such dryer hoods is intended to be included by the invention.

FIG. 9 diagrammatically illustrates heat flow through the outer cylindrical surface of a Yankee dryer in a typical papermaking operation wherein two successive pressure rolls are arranged in contact with the surface of the Yankee dryer with a felt running between the rolls and the dryer, and a felt loop is formed by a guide roll disposed between the pressure rolls and spaced from the dryer surface. In addition, a dryer hood is employed over the major portion of the dryer surface extending from a point adjacent the second pressure roll to a point adjacent the creping doctor blade. The graph lines represent transient heat flow conditions existing in a very thin outer layer adjacent the outer surface of the dryer shell as heat flow conditions in the inner portion of the dryer shell are substantially steady state. For comparison purposes, the above heat flow conditions are presented both for conventional apparatus and for an arrangement of apparatus similar to that shown in FIGS. 7 and 8. From this it can be seen that with the latter, heat which is retained in the surface of the Yankee dryer shell as it moves through the first drying nip to the second drying nip and through that nip, is available for drying the web on the Yankee dryer when all the pressing operations have been completed.

Referring now to FIG. 9, the ordinate represents heat flow through the outer layer of the dryer shell in thousands of B.t.u.s/hour/ft. and the abscissa represents distance in inches along the circumference of the dryer shell from the point Where the wet web first contacts the dryer surface to the point where the web is removed from the dryer surface. The solid line represents transient heat flow conditions in a conventional arrangement of apparatus while the dotted line represents heat flow in the apparatus of the invention as described above.

The letter A represents the circumferential length of dryer surface in pressure contact with the first pressure roll. The letter B represents the circumferential length of dryer surface between termination of contact with the first pressure roll and commencement of contact with the second pressure roll. The letter C indicates the circumferential length of dryer surface in contact with the second pressure roll at the second drying nip. The letter D illustrates the circumferential length of dryer surface in contact with the wet web extending from the end of the second drying nip to the creping doctor blade or point where the web is removed from the dryer surface. The major portion of the length D lies beneath a high velocity dryer hood which promotes drying both by the circulation of air over the surface of the wet web carried on the dryer surface and by the removal of evaporated moisture or water vapor therefrom. During the travel of a point on the dryer surface from the end of length D around to the start of length A, the shell stores heat and the temperature of the outer surface rises.

From the above diagram it can be seen that with the conventional arrangement of apparatus, considerable heat is lost from the dryer surface in the first drying nip, A, the second drying nip, C, and the space B between the first and second drying nips. Of course, a large portion is also lost along length D of the dryer circumference during evaporative drying. With apparatus of the invention, however, loss of heat at the first and second drying nips, A and C, and in the intervening space, B, between the drying nips is prevented. This heat is available for use in drying the web when it passes through the position shown as length D of the dryer circumference. The heat is more efficiently used at this point for evaporative drying as the temperature of the dryer surface is higher and the heat can be used to elevate the web and the moisture 12 therein to a temperature where the moisture vaporizes. When the heat is lost to the web and the felt in the first and second drying nips and the space between these nips, it is inefficiently applied, since the web and felt are quite cold and the heat is not transferred long enough to raise much moisture to vaporization temperature.

What is claimed is: 1. A method for drying a freshly-formed wet paper Web, comprising:

pressing one surface of said web into contact with a traveling felt,

transporting said web, in supported condition upon sa1d traveling felt and free from any interim compression, directly to a cylindrical heat-conductive drying surface,

contacting the felt-opposite surface of said Web with steam immediately prior to contact of said feltopposite surface with said drying surface, by emitting steam from steam supply means extending across the width of said web and disposed between two seal plates, one of said seal plates extending from said steam supply means to a position closely spaced from said web, the other of said seal plates extending from said steam supply means to a position closely spaced from said drying surface,

drawing said steam into said Web and generally through said web and drawing entrained moisture from said web onto said traveling felt by subjecting the feltfacing surface of said web to a partial vacuum immediately prior to contact of said felt-opposite surface with said drying surface,

pressing the felt-opposite surface of said web into contact with said drying surface, and removing said web from said drying surface.

2. A method for drying a freshly-formed Wet paper web according to claim 1, wherein said cylindrical heat-conductive drying surface is the surface of a Yankee dryer, and said steam is superheated when it contacts the felt- .opposite surface of said web.

3. In the operation of a papermaking machine of the single-felt type, wherein a freshly-formed wet paper web is couched from the surface of the forming wire by a traveling pickup felt, advanced in supported condition free from any interim compression on said pickup felt directly to the surface of a Yankee dryer, and pressed into engagement with the surface of said Yankee dryer by at least one suction pressure roll, the improvement comprising:

applying steam to the felt-opposite surface of said web as said web passes through a pre-nip region and into the pressure nip between said Yankee dryer and the first suction pressure roll encountered, by emitting steam from steam supply means extending across the width of said web and disposed between two seal plates, one of said seal plates extending from said steam supply means to a position closely spaced from said web, the other of said seal plates extending from said steam supply means to a position closely spaced from said Yankee dryer, and

applying a vacuum bias to the felt-facing surface of said web.

4. In a method of reducing the moisture in a freshlyformed wet paper web, wherein said web in contact with a traveling felt is passed through a drying nip between a Yankee dryer and a pressure roll, the improvement comprising:

simultaneously contacting the felt-opposite surface of said web with steam while applying a partial vacuum to the felt-facing surface of said web in successive transverse portions of said web preceding said drying p,

and maintaining the application of steam to the feltopposite surface of said web until passage of said Web into said drying nip by emitting steam from steam supply means extending across the width of said web and disposed between two seal plates, one of said seal plates extending from said steam supply means to a position closely spaced from said web, the other of said seal plates extending from said steam supply means to a position closely spaced from said Yankee dryer.

5. In a method of reducing the moisture in a freshlyformed wet paper web, wherein said web in contact with a traveling felt is passed through a drying nip between a Yankee dryer and a solid pressure roll, the improvement comprising:

flowing steam along one surface of said web in a direction opposite to the direction of travel of said web and from a point adjacent the on-running side of said drying nip to a point spaced therefrom and preceding said pressure roll, by emitting steam from steam supply means extending across the width of said web and disposed between two seal plates, one of said seal plates extending from said steam supply means to a position closely spaced from said web along a line preceding said pressure roll, the other of said seal plates extending from said steam supply means to a position closely spaced from said Yankee dryer, and

elevating the temperature of each of said web, said felt, and moisture contained therein, prior to their passing through said drying nip, by drawing said steam through successive transverse portions of said traveling web and said traveling felt preceding and moving toward said pressure roll.

6. In a method of reducing the moisture in a freshlyformed wet paper web, wherein said web in contact with a traveling felt is passed through at least two successive drying nips between a Yankee dryer and two successive pressure rolls, the improvement comprising:

separating said web and said felt from each other between said two successive drying nips to create a space and to expose the portion of said web carried in contact upon the surface of said Yankee dryer, and inserting steam from a remote source into said space.

7. The improvement in the method according to claim 6- including the steps of:

simultaneously contacting the felt-opposite surface of said web with steam while applying a partial vacuum to the felt-facing surface of said web in successive transverse portions of said web preceding the first of said two successive drying nips, and

maintaining the application of steam to the felt-opposite surface of said web until passage of said web into said first drying nip.

8. In combination in a papermaking machine:

a rotatably mounted drum dryer having a cylindrical heat-conductive drying surface,

a rotatably mounted suction pressure roll disposed adjacent said drying surface, the rotational axis of said pressure roll being substantially parallel to the rotational axis of said drum dryer, said pressure roll forming a drying nip with said drying surface,

vacuum means for reducing the pressure on the interior of said suction pressure roll,

a traveling felt partially wrapping said pressure roll, said felt running along a path through said drying nip between said pressure roll and said drying surface in a direction substantially perpendicular to said drying nip and conveying a paper web from a remote position to said drying surface, and

steam supply means disposed adjacent the on-running side of said drying nip into which said felt and web are advanced, said steam supply means including a header introducing steam adjacent the on-running side of said drying nip, and two seal plates substantially co-extensive with said header, one of said seal plates extending from said header to a position closely spaced from said web, the other of said seal plates extending from said header to a position closely spaced from said drying surface, said header having a plurality of openings through the portion of its sidewall facing said drying nip and between said seal plates, said steam supply means enveloping with steam a portion of said drying surface and a portion of the paper web carried on the surface of said felt.

9. The combination according to claim 8, wherein said steam supply means extends across the entire width of said paper web and emits steam at a substantially uniform velocity along its length.

10. The combination in a papermaking machine according to claim 8, including a second rotatably mounted pressure roll disposed adjacent said drying surface and spaced farther along the path of felt travel from the offrunning side of said drying nip, the rotational axis of said second pressure roll being substantially parallel to the rotational axis of said drum dryer, said second pressure roll forming a second drying nip with said drying surface, said felt running through said second drying nip between said second pressure roll and said drying surface in contact with the web carried on said drying surface.

11. The combination in a papermaking machine according to claim 10, including:

a guide roll about which the portion of said traveling felt between the first and second drying nips runs, said guide roll forming a felt loop separated and spaced from the paper web carried on said drying surface, and exposing one surface of said paper web running between the first and second drying nips,

hood means substantially enclosing the portion of said paper web so exposed between the first and second drying nips, and

duct means for introducing steam into said hood means and into contact with the enclosed portion of said paper web.

12. The combination in a papermaking machine according to claim 10, including:

a guide roll about which runs the portion of said traveling felt between the first and second drying nips, said guide roll forming a felt loop separated and spaced from the paper web carried on said drying surface, and exposing one surface of said paper web running between the first and second drying nips,

two spaced-apart end plates disposed adjacent each side of said felt loop and closely spaced relative to said felt and said drying surface, said end plates cooperating with said felt loop and said drying surface to form a chamber substantially enclosing the portion of said paper web so exposed between the first and second drying nips, and

duct means for introducing steam into said chamber through at least one of said end plates and into contact with the enclosed portion of said paper web.

13. In combination in a papermaking machine:

a rotatably mounted drum dryer having a cylindrical heat-conductive drying surface,

first and second rotatably mounted pressure rolls disposed adjacent said drying surface, the rotational axes of said first and second pressure rolls being substantially parallel to the rotational axis of said drum dryer, said first and second pressure rolls forming successive first and second drying nips, respectively, with said drying surface,

a traveling felt running along the path through said first and second drying nips in a direction substantially perpendicular to said drying nips, said felt being adapted to convey a paper web from a remote position to said drying surface,

a guide roll about which runs the portion of said traveling felt between the first and second drying nips, said guide roll forming a felt loop separated and spaced from the paper web carried on said drying surface and exposing one surface of said paper web carried 15 on the drying surface between the first and second drying nips,

hood means substantially enclosing the portion of said paper web so exposed between the first and second drying nips, and

duct means for introducing steam into said hood means and into contact with the enclosed portion of said paper web.

14. The combination in a paperma'king machine according to claim 13, wherein said hood means comprise two spaced-apart end plates in cooperative arrangement with said felt loop, said end plates being disposed adjacent each side of said felt loop and closely spaced relative to said felt and said drying surface, said end plates cooperating with said felt loop and said drying surface to form a chamber substantially enclosing the portion of said paper web so exposed between the first and second drying nips, and said duct means lead into said chamber through at least one of said end plates.

15. The combination in a papermaking machine according to claim 8, wherein said openings are directed generally toward said seal plates and emit steam from the interior of said header and impinge said steam off of said seal plates to uniformly diffuse it into the space between said seal plates.

16. The combination in a papermaking machine according to claim 8, wherein said suction pressure roll is a solid pressure roll, said vacuum means are disposed adjacent said felt opposite said steam supply means, said vacuum means drawing said steam through successive transverse portions of said traveling web and said felt which precede and are moved toward said pressure roll, and said steam supply means include a header for introducing steam adjacent the on-running side of said drying nip, and two seal plates substantially co-extensive with said header, one of said seal plates extending from said header to a position closely spaced from said web, the other of said seal plates extending from said header to a position closely spaced from said drying surface, said header having a plurality of openings through the portion of its sidewall facing said drying nip and between said seal plates, said openings being directed generally toward said seal plates and emitting steam from the interior of said header and impinging said steam off of said seal plates to uniformly diffuse it into the space between said seal plates.

17. In combination in a papermaking machine:

a rotatably mounted drum dryer having a cylindrical heat-conductive drying surface,

a rotatably mounted solid pressure roll disposed adjacent said drying surface, the rotational axis of said pressure roll being substantially parallel to the rotational axis of said drum dryer, said pressure roll forming a drying nip with said drying surface,

1 6 a traveling felt partially wrapping said pressure roll, said felt running through said drying nip between said pressure roll and said drying surface in a direc tion substantially perpendicular to said drying nip, said felt conveying a paper web from a remote position to said cylindrical drying surface.

hood means extending from the on-running side of said drying nip to a point spaced therefrom and preceding said pressure roll, said hood means substantially enclosing successive transverse portions of said Web carried on said traveling felt and including two seal plates substantially co-extensive with said felt, one of said seal plates extending from said hood means to a position closely spaced from said web along a line preceding said pressure roll, the other of said seal plates extending from said hood means to a position closely spaced from said drying surface,

steam supply means connected to said hood means for introducing steam at an elevated temperature into said hood means, in a position closely spaced to said drying nip, and

suction means disposed adjacent said felt opposite said hood means, said suction means drawing said steam through successive transverse portions'of said traveling web and said felt which precede and are moving toward said pressure roll.

18. In a papermaking machine the combination ac cording to claim 17, wherein said pressure roll is a blind drilled roll.

19. 'In a papermaking machine the combination according to claim 17, wherein said pressure roll is a grooved roll.

References Cited UNITED STATES PATENTS 1,925,917 9/1933 Chalon 162-358 2,443,352 6/1948 Hornbostel 162359 2,672,078 3 1954 Hornbostel 162360 2,732,772 1/1956 Hornbostel 162-359 2,907,690 10/1959 Hornbostel 162358 3,097,994 7/1963 Dickens 162-359 3,198,695 8/1965 Justus 162-360 FOREIGN PATENTS 682,966 3/1964 Canada 162359' S. LEON BASHORE, Primary Examiner R. H. ANDERSON, Assistant Examiner US. Cl. X.R. 

